1.4.1 Scalability Improvements

MySQL 5.5 incorporates changes in
InnoDB that improve the performance
of RW-locks by using atomic CPU instructions (on platforms where
they are available), rather than less scalable mutexes. It is
also possible for InnoDB memory
allocation to be disabled and replaced by the normal
malloc library, or by a different library
that implements malloc such as
tcmalloc on Linux or
mtalloc on Solaris.

The reimplementation of RW-locks requires atomic instructions. A
status variable,
Innodb_have_atomic_builtins,
shows whether the server was built with atomic instructions.